The present invention relates generally to alternating current (AC) motors and, more particularly, to a system and method for determining stator winding resistance for thermal protection of AC motors.
Thermal protection is an important aspect in the monitoring of motor conditions, as motor failures can often be related to the stator winding insulation. It is commonly assumed that the motor's life is reduced by 50% for every 10° C. increase in temperature above an acceptable stator winding temperature limit. Therefore, accurate monitoring of the stator winding temperature is beneficial for motor protection purposes.
Various methods and mechanisms for determining the stator winding temperature are currently employed. Aside from the direct stator winding temperature measurement, thermal model-based and motor parameter-based temperature estimation methods are two major techniques for thermal protection. The thermal model-based methods estimate the stator winding temperature using motor thermal models. However, due to the thermal parameter variation and the difficulty of thermal parameter identification, the accuracy of these methods can fall outside acceptable ranges. Besides, due to the change of cooling conditions, the thermal parameters are not constant, and may need to be identified for each motor under each specific cooling condition.
The AC motor parameter-based approaches to determining stator winding temperature are developed to estimate the average winding temperatures from the stator winding resistances. Stator winding resistance (Rs) estimation is broadly used not only in thermal protection, but also in motor controls, fault diagnosis, and efficiency evaluation. Over the years, various Rs estimation methods have been proposed for different purposes. Generally, they are divided into three major categories: direct measurement methods, equivalent circuit-based methods, and signal-injection-based methods. Direct methods, such as the IEEE standard-118, give the most accurate Rs estimates, but have limitations and drawbacks due to the fact that resistance is only measured at a certain temperature and the resistance variations due to temperature changes are not considered. A further drawback of direct measurement methods is that the motor has to be disconnected from service to perform the required tests.
The equivalent circuit-based methods of Rs estimation use the motor current and voltage to calculate the stator resistance based on an AC motor equivalent circuit (i.e., a model of the AC motor). Such model-based methods are non-intrusive and can respond to changes in the cooling conditions but are generally too sensitive to motor parameter variations to provide useful results for temperature estimation. That is, the estimation error of model-based methods can be about 20%, which is too high for use in temperature estimation.
The signal injection-based methods for determining stator resistance inject a DC bias into the stator supply voltage and use the DC component of the voltage and current to calculate the stator resistance. In one DC injection method, a resistor in parallel with a transistor is installed in one phase of the motor, which leads to an equivalent resistance in the induction motor that is different when input current is positive and negative, thus producing a DC component. Although this approach can be accurate and robust to the variations in cooling conditions and motor parameters, it suffers from its intrusive nature, as an extra DC injection circuit needs to be installed in series with one of the motor leads. Additionally, due to the current limits of semiconductor devices, previous signal injection-based methods cannot generally be directly applied to motors beyond 100 hp.
It would therefore be desirable to design an accurate, non-intrusive method for determining stator winding resistance, without adding further resistance to the induction motor.